Hi Bob,
Voids in BGA solder balls (near eutectic, not high melt) after reflow are
normal, but not before reflow.
Voids in the balls prior to reflow reduce the available solder volume, which
may cause opens if large enough.
Voids after reflow are caused by flux/flux action volatiles being trapped by
the horizontal component pads and occur because most BGA solder balls are
barely long enough in the liquid state to produce adequate wetting (and
sometimes not).
Voids are typically in the upper half (near the component) of the balls.
All indications are that the typically occurring voids (not too large, and
not porosities near the interface) do not present a reliability problem; in
fact there are some studies that show some (not significant enough to go for
voids on purpose) increase in life. The possible reasons for this are 1) an
increased solder joint height for a fixed solder volume, 2) the localized and
temporary retardation of the crack propagation by the voids (a la Liberty
Bell).
However, if you need thermal conduction or electrical current carrying
capacity, you definitely do not want voids.
Higher temperatures and longer times in the liquid state will reduce voids,
as well some fluxes/pastes.

Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL  32174  USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask], Website: www.engelmaier.com

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